I'm doing a split income return with my wife. Some of her details are tbd but we can estimate. Can I file mine with hers to follow, or must we wait until both are final?

You can file yours before her return is finished, but you do run a risk of being reassessed. That could mean less of a refund (or more, depending on the changes CRA makes), interest penalties if the CRA needs to change amounts, and you end up owing taxes instead of getting a refund.

If you think the numbers are close enough, then there is nothing preventing you from filing. Just be aware of the follow-up actions in case the CRA has any questions or wants to see documents/receipts.